For reasons of space requirements, weight, and cost, manufacturers are led increasingly to reduce the dimensions of electric motors. This greatly increases the need to protect the electric motor from excessive heating which would be fatal to it (damage to the brushes, separation of the commutator segments, short-circuiting of the armature, etc.).
In order to avoid the specific problems of space requirements mentioned above which would be further increased if the thermal protection were incorporated inside the motors, manufacturers have gradually turned to the study of thermal protection arranged outside the motors. Such an arrangement can also prevent manufacturers from having to individually customize the motors according to the different specifications presented by their clients during assembly, and therefore from having to manage a great stock of different parts for the production line.
An example of external thermal protection is given in Patent No. GB-A-2,668,661, in an internal combustion engine starter application. Essentially, a thermal switch is thermally coupled to the ferrule of a power supply cable of the electric starter, wherein the ferrule size is chosen so that the heat produced in the ferrule is proportional to the internal temperature of the starter. The thermal switch is capable of cutting the power supply of the starter when the temperature of the cable ferrule exceeds an index temperature corresponding to a maximum authorized temperature within the starter.
For the same specific reasons of space requirements, manufacturers have also turned towards anti-interference devices for electric motors arranged outside of the motors. For this purpose, it is known to use in particular capacitors and/or anti-interference coils intended to filter the interference frequencies. In a widespread form of execution, the anti-interference coil consists of a solenoid with a coil wound on a cylindrical ferrite core.
All the protective devices mentioned above are generally satisfactory. However, it would be desirable to be able to obtain protection from electromagnetic interference and thermal protection in a simpler manner using more compact means, due to the ever-shrinking size requirements for electric motors. The present invention is directed toward meeting this need.